Semiconductor chip with anti-reverse engineering function

ABSTRACT

A structure and a method. The structure includes a semiconductor substrate; a stack of wiring levels from a first wiring level to a last wiring level, the first wiring level closest to the semiconductor substrate and the last wiring level furthest from the semiconductor substrate, the stack of wiring levels including an intermediate wiring level between the first wiring level and the last wiring level; active devices contained in the semiconductor substrate and the first wiring level, each wiring level of the stack of wiring levels comprising a dielectric layer containing electrically conductive wire; a trench extending from the intermediate wiring level, through the first wiring level into the semiconductor substrate; and a chemical agent filling the trench, portions of at least one wiring level of the stack of wiring levels not chemically inert to the chemical agent or a reaction product of the chemical agent.

This application is a divisional application claiming priority to Ser.No. 14/862,700 filed Sep. 23, 2015.

BACKGROUND

The present invention relates to the field of integrated circuits; morespecifically, it relates to semiconductor chips with anti-reverseengineering functions and methods of fabricating semiconductor chipswith anti-reverse engineering functions.

Semiconductor chips often contain intellectual property or sensitivestructures that can be reverse-engineered resulting in the potentialloss of such information or the disclosure of sensitive information.Accordingly, there exists a need in the art to mitigate the deficienciesand limitations described hereinabove.

SUMMARY

A first aspect of the present invention is a structure, comprising: asemiconductor substrate; a stack of wiring levels from a first wiringlevel to a last wiring level, the first wiring level closest to thesemiconductor substrate and the last wiring level furthest from thesemiconductor substrate, the stack of wiring levels including anintermediate wiring level between the first wiring level and the lastwiring level; active devices contained in the semiconductor substrateand the first wiring level, each wiring level of the stack of wiringlevels comprising a dielectric layer containing electrically conductivewire; a trench extending from the intermediate wiring level, through thefirst wiring level into the semiconductor substrate; and a chemicalagent filling the trench, portions of at least one wiring level of thestack of wiring levels not chemically inert to the chemical agent or areaction product of the chemical agent.

A second aspect of the present invention is a structure, comprising: asemiconductor substrate; a stack of wiring levels from a first wiringlevel to a last wiring level, the first wiring level closest to thesemiconductor substrate and the last wiring level furthest from thesemiconductor substrate, the stack of wiring levels including anintermediate wiring level between the first wiring level and the lastwiring level; active devices contained in the semiconductor substrateand the first wiring level, each wiring level of the stack of wiringlevels comprising a dielectric layer containing electrically conductivewire; a first trench extending from the intermediate wiring level,through the first wiring level into the semiconductor substrate; asecond trench extending from the intermediate wiring level, through thefirst wiring level into the semiconductor substrate; and a firstchemical agent filling the first trench and a second chemical agentfilling the second trench, portions of at least one wiring level of thestack of wiring levels not chemically inert to a reaction product of thefirst chemical agent and the second chemical agent or can be physicallydamaged by the reaction product.

A third aspect of the present invention is a method, comprising:providing a semiconductor substrate; forming a stack of wiring levelsfrom a first wiring level to an intermediate wiring level, the firstwiring level closest to the semiconductor substrate and the intermediatewiring level furthest from the semiconductor substrate; forming activedevices contained in the semiconductor substrate and the first wiringlevel, each wiring level of the stack of wiring levels comprising adielectric layer containing electrically conductive wire; forming one ormore trenches extending from the intermediate wiring level, through thefirst wiring level into the semiconductor substrate; (i) filling the oneor more trenches with a first chemical agent that can cause damage to ordestroy portions of at least one wiring level of the stack of wiringlevels or (ii) filling a first group of the one or more trenches with asecond chemical agent and a filling a second group of the one moretrenches a third chemical agent, a mixture of the second chemical agentand the third chemical agent generating a fourth chemical agent that cancause damage to or destroys portions of at least one wiring level of thestack of wiring levels; forming caps on the one or more trenches; andforming one or more additional wiring levels on top of the intermediatewiring level.

These and other aspects of the invention are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention are set forth in the appended claims. Theinvention itself, however, will be best understood by reference to thefollowing detailed description of illustrative embodiments when read inconjunction with the accompanying drawings, wherein:

FIG. 1 is a cross-section of an exemplary integrated circuit containingan anti-reverse engineering structure according embodiments of thepresent invention;

FIG. 2 is a cross-section of an exemplary integrated circuit containinga binary anti-reverse engineering structure according embodiments of thepresent invention;

FIG. 3 is a cross-section of an exemplary integrated circuit containinga nested binary anti-reverse engineering structure according embodimentsof the present invention;

FIG. 4 is cross-section through line 4-4 of FIG. 1;

FIG. 5 is cross-section through line 5-5 of FIG. 2;

FIG. 6 is a cross-section through line 6-6 of FIG. 3;

FIGS. 7A through 7D illustrate a method of fabricating an integratedcircuit containing an anti-reverse engineering structure accordingembodiments of the present invention;

FIGS. 8A and 8B illustrate alternative steps forming the trench of FIG.7A;

FIG. 9 is an exemplary cross-section of the sidewall layer of thetrenches of FIGS. 1, 2 and 3 according to embodiments of the presentinvention;

FIG. 10 is a cross-section illustrating the action of anti-reverseengineering structures according to embodiments of the present inventionwhen the semiconductor chip is delayered from the top surface down; and

FIG. 11 is a cross-section illustrating the action of anti-reverseengineering structures according to embodiments of the present inventionwhen the semiconductor chip is cross-sectioned.

DETAILED DESCRIPTION

The anti-reverse engineering structures according to the embodiments ofthe present invention comprise sealed trenches completely within theactive and wiring levels of a semiconductor chip. The sealed trenchescontain chemical agents that will damage or destroy one or more of thematerials in the various layers of the semiconductor chip and/or thesemiconductor substrate by chemical attack or physical or thermal stresswhen the seal around the trench is broken, thereby preventing reverseengineering or making reverse engineering very difficult.

The anti-reverse engineering structures according to embodiments of thepresent invention are not formed above the wiring levels of the chip andare not formed in the bottom side of the semiconductor substrate (theside opposite from the top side of the semiconductor chip where activedevices such as transistors are formed). Rather the sealed trenchescontaining the chemical agents are contained completely within theactive and wiring levels of a semiconductor chip making them difficultto detect as they not visible. One or more of the materials of thesemiconductor chip are not chemically inert to chemical agents or arenot chemically inert to chemical agents generated by the reaction of thechemical agent with air, oxygen or water, or are not chemically inert toa chemical agent generated by the mixing of two different chemicalagents contained in different trenches. The anti-reverse engineeringstructures according to embodiments of the present invention are passivein that they do not require heat or electrical ignition to activate.

FIG. 1 is a cross-section of an exemplary integrated circuit containingan anti-reverse engineering structure according embodiments of thepresent invention. In FIG. 1, semiconductor chip 100 includes asemiconductor substrate (i.e., a silicon or silicon on insulator) andfirst dielectric layer 110. First dielectric layer 110 may itself beformed from two or more dielectric layers examples of which silicondioxide (SiO₂), silicon nitride (Si₃N₄), silicon carbide (SiC), siliconoxy nitride (SiON), silicon oxy carbide (SiOC), plasma-enhanced siliconnitride (PSiN_(x)) and Borophosphosilicate glass (BPSG). Formed insubstrate 105 and first dielectric layer 110 are field effecttransistors (FETs) 106 which comprise source/drains 107 and 108 andgates 109. FETs 106 are electrically contacted by wires 115 in firstdielectric layer 110.

Formed in a stack on first dielectric layer 110 are a series of wiringlevels containing electrically conductive wires embedded incorresponding dielectric layers. Second dielectric layer 120 containswires 125, third dielectric layer 130 contains wires 135, fourthdielectric layer 140 contains wires 145, fifth dielectric layer 150contains wires 155, sixth dielectric layer 160 contains wires 165,seventh dielectric layer 170 contains wires 175, eighth dielectric layer180 contains wires 185 and ninth dielectric layer 190 containselectrically conductive pads 195. Formed on top of ninth dielectriclayer is a dielectric passivation layer 200. An electrically conductivepad limiting metallurgy (PLM) layer 205 is formed in openings inpassivation layer 200 and electrically conductive solder bumps 210 areformed on PLM layers 205. Wires 110, 120, 130, 140, 150, 160, 170, 180and pads 195 interconnect FETs 106 into integrated circuits and provideinput/output (I/O) and power connections to semiconductor chip 100through solder bumps 210.

In one example, dielectric layers 110, 120, 130, 140, 150, 160 170, 180and 190 are themselves comprised of two or more dielectric layers. Inone example, dielectric layers 110, 120, 130, 140, 150, 160 170, 180 and190 independently comprise SiO₂, Si₃N₄, high density plasma (HDP) oxide,tetraethyl orthosilicate (TEOS) chemical vapor deposited (CVD) SiO2,hydrogen silsesquioxane polymer (HSQ), methyl silsesquioxane polymer(MSQ), methyl doped silica or SiO_(x)(CH₃)_(y) or SiC_(x)O_(y)H_(y) orSiOCH, organosilicate glass (SiCOH), and porous SiCOH. In one example,passivation layer 200 comprises polyimide. In one example, wires 115,125, 135, 145, 155, 165, 175, 185 and pads 195 independently compriseone or more layers of copper (Cu) , tungsten (W), aluminum (Al) aluminumcopper (AlCu), aluminum copper silicon (AlCuSi), titanium (Ti), tantalum(Ta), tungsten nitride (WN), titanium nitride(TiN) and tantalum nitrideTaN).

Formed in semiconductor substrate 105 and extending through dielectriclayers 110, 120, 130, 140, 150 and 160, but not extending intodielectric layer 170, is a trench 215. Formed on the sidewalls 217 andbottom 218 of trench 215 is a liner 220. Liner 220 may be formed fromone or more layers. (See FIG. 9 and description infra for an example).Formed in dielectric layer is a cap 225 that seals the top of trench215. Cap 225 may extend into trench 215. Cap 225 may be formed from oneor more layers. Cap 225 may be formed from the same materials as liner220. In one example, liner 220 and cap 225 independently comprise one ormore layers of gold (Au), platinum (Pt), Ti, Ta, Si₃N₄, a stressabsorption layer and TEOS oxide5. Remaining space (that space not takenup by liner 220) in trench is either completely filled or partiallyfilled (to line 230) with an agent 235. Partially filling trench 215allows for expansion of agent 235 when integrated circuit chip issubjected to heat during fabrication, in one example, up to 430° C.Agent 235 will (i) chemically attack one or more of the materials in thevarious layers of the semiconductor chip and/or the semiconductorsubstrate or (ii) generate an agent that will chemically attack one ormore of the materials in the various layers of the semiconductor chipand/or the semiconductor substrate when exposed to air (i.e., the oxygenin air) or water. Liner 220 and cap 225 are not attacked by agent 235.

In one example, agent 235 comprises a material that reacts with oxygen.In one example, agent 235 comprises a material that reacts with water.In one example, agent 235 comprises boron trichloride (BCl₃) whichreacts with water to form HCl. In one example, agent 235 comprisestrichlorosilane (SiCl₃H) which reacts with air to form HCl. In oneexample, agent 235 comprises iron (Fe) or aluminum (Al) in an oxygenfree slurry which will reacts with air exothermically. In one example,agent 235 comprises hydrofluoric acid (HF) or buffered HF (BHF),hydrochloric acid (HCl), phosphoric acid (H3PO4), nitric acid (HNO3) orpotassium hydroxide (KOH). In one example, agent 235 comprises sodiumpolyacrylate.

While nine wiring levels are illustrated in FIG. 1, there may be lessthan nine or more than nine. While trench 215 is illustrated extendingfrom within substrate 105 up through dielectric layer 170, trench 215may extend into layers above dielectric layer 160 or only to a layerbelow dielectric layer 160. The terms above and below are defined in theframe of reference where silicon substrate 105 is the bottom of thesemiconductor chip and passivation layer 200 is the top of thesemiconductor chip. While one filled and capped trench 215 isillustrated in FIG. 1, there may be multiple filled and capped trenchesdistributed throughout the semiconductor chip. Multiple filled andcapped trenches may be disturbed individually or in groups of tens tohundreds. Groups of filled and capped trenches may be distributedthroughout the semiconductor chip or in specific regions of thesemiconductor chip that are deemed sensitive.

FIG. 2 is a cross-section of an exemplary integrated circuit containinga binary anti-reverse engineering structure according embodiments of thepresent invention. Semiconductor chip 100A of FIG. 2 is similar tosemiconductor chip 100 of FIG. 1. In FIG. 2, trench 215 of FIG. 1 isreplaced with a pair of trenches, a first trench 215A and a secondtrench 215B. First trench 215A includes a first liner 220A, a first cap225A and a first agent 235A that either completely fills remaining space(that space not taken up by liner 220A) in trench 215A or fills theremaining space in trench 215A to line 230A. Second trench 215B includesa second liner 220B, a second cap 225B and a second agent 235B thateither completely fills remaining space (that space not taken up byliner 220B) in trench 215B or fills the remaining space in trench 215Bto line 230B. First liner 220A and first cap 225A are not attacked byagent 235A. Second liner 220B and second cap 225B are not attacked byagent 235B. The materials of first liner 220A and second liner 220B arethe same as for liner 220 of FIG. 1 described supra. The materials offirst cap 225A and second cap 225B are the same as for cap 225 of FIG. 1described supra.

First agent 235A and second agent 235B form a binary system thatgenerate, when mixed together, a reaction product that will (i) generatea chemical agent that will chemically attack one or more of thematerials in the various layers of the semiconductor chip and/or thesemiconductor substrate or (ii) physically or thermally damage thesemiconductor chip when exposed, for example, by a increase in volume.In one example, first agent 235A comprises an isocyanate and secondagent 235B comprises a polyol. In one example, first agent 235Acomprises BC1 ₃ and second agent 235B is water. In one example, firstagent 235A comprises sodium polyacrylate and second agent 235B is wateror a water containing gel.

FIG. 3 is a cross-section of an exemplary integrated circuit containinga nested binary anti-reverse engineering structure according embodimentsof the present invention. Semiconductor chip 100B of FIG. 3 is similarto semiconductor chip 100 of FIG. 1. In FIG. 3, trench 215 of FIG. 1 isreplaced with a pair of trenches, a third trench 215C and a fourthannular trench 215D surrounding third trench 215C. Third trench 215C isseparated from fourth trench 215D by a ring of stacked semiconductorsubstrate 105, first dielectric layer 110, second dielectric layer 120,third dielectric layer 130, fourth dielectric layer 150 and sixthdielectric layer 160. Third trench 215C includes a third liner 220C, athird cap 225C and a third agent 235C that either completely fillsremaining space (that space not taken up by liner 220C in trench 215C orfills the remaining space in trench 215C to line 230C. Fourth trench215D includes a fourth liner 220D, a fourth cap 225D and a fourth agent235D that either completely fills remaining space (that space not takenup by liner 220D) in trench 215D or fills the remaining space in trench215D to line 230D. Third liner 220C and third cap 225C are not attackedby agent 235C. Fourth liner 220D and fourth cap 225D are not attacked byagent 235D. The materials of third liner 220C and fourth liner 220D arethe same as for liner 220 of FIG. 1 described supra. The materials ofthird cap 225C and fourth cap 225D are the same as for cap 225 of FIG. 1described supra.

Third agent 235C and fourth agent 235D form a binary system thatgenerate, when mixed together, a material a material will (i) generatean agent that will chemically attack one or more of the materials in thevarious layers of the semiconductor chip and/or the semiconductorsubstrate or (iii) physically or thermally damage the semiconductor chipwhen exposed, for example, by a increase in volume. In one example,third agent 235C comprises an isocyanate and fourth agent 235D comprisesa polyol. In one example, third agent 235C comprises BC1 ₃ and fourthagent 235D is water. In one example, third agent 235C comprises sodiumpolyacrylate and fourth agent 235D is water or a water containing gel.

FIG. 4 is cross-section through line 4-4 of FIG. 1. In FIG. 4, trench215 has a diameter D1 and liner 220 has a thickness T1. In one example,D1 is between 1 um and 10 um. In one example, T1 is between 10 nm and100 nm.

FIG. 5 is cross-section through line 5-5 of FIG. 2. In FIG. 5, trench215A has a diameter D2 and trench 215B has a diameter D3. Liner 220A hasa thickness T2 and liner 220B has a thickness T3. In one example, D2 isbetween 1 um and 10 um. In one example, D3 is between lum and 10 um Inone example, T2 is between 10 nm and 100 nm In one example, T3 isbetween 10 nm and 100 nm

FIG. 6 is a cross-section through line 6-6 of FIG. 3. In FIG. 6, trench215C has a diameter D4 and trench 215D has an inside diameter D5 and anoutside diameter D6. Liner 220C has a thickness T4 and liner 220D has athickness T5. In one example, D4 is between 0.5 um and 5 um. In oneexample, D5 is between 1 um and 10 um. In one example, D6 is between 2um and 20 um. In one example, T4 is between 10 nm and 100 nm. In oneexample, T5 is between 10 nm and 100 nm.

FIGS. 7A through 7D illustrate a method of fabricating an integratedcircuit containing an anti-reverse engineering structure accordingembodiments of the present invention. In FIGS. 7A through 7B, thestructure illustrated in FIG. 1 is used as an example. In FIG. 7A,semiconductor chip 100 is formed through dielectric layers 160 and atrench 215 is formed through dielectric layers 110, 120, 130, 140, 150,160 into substrate 105. There were no wires 115, 125, 135, 145, 155, 165in the dielectric layers in the regions of the dielectric layers thattrench 215 was formed in. In FIG. 7B, liner 220 is formed. In oneexample, liner 220 is also formed on the top surface of dielectric layer160. In FIG. 7C, liner 220 is removed from the top surface of dielectriclayer 160 using, for example, a directional etch. In FIG. 7D, trench 215is filled with agent 235 and cap 225 formed. The additional layers,wires and structures on top of layer dielectric 160 illustrated in FIG.1 are next formed.

FIGS. 8A and 8B illustrate alternative steps forming the trench of FIG.7A. In FIG. 8A, semiconductor chip 100 is completed only throughdielectric layer 110 and a trench 245 formed from the top surface ofdielectric layer 110 into substrate 105. There are no wires 115 or FETs106 (or other devices) in the region of dielectric layer 110 andsubstrate 105 in which trench 245 is formed. In FIG. 8B, semiconductorchip is completed through dielectric layer 160. Trench 245 is filledwith dielectric material from one or more subsequent layers (not shown)subsequently removed when trench 215 is formed through dielectric layers120, 130, 140, 150, 160 into substrate 105. There were no wires 125,135, 145, 155, 165 in the dielectric layers in the regions of thedielectric layers that trench 215 was formed in. The method nextproceeds to FIG. 7B.

FIG. 9 is an exemplary cross-section of the sidewall layer of thetrenches of FIGS. 1, 2 and 3 according to embodiments of the presentinvention. In FIG. 9 liner 220 comprises a buffer layer 250 on sidewall217 of trench 215. A stress absorption layer 255 is formed on bufferlayer 250, a water barrier layer 260 is formed on stress absorptionlayer 255, an adhesion layer 265 is formed on water barrier layer 260and a chemically resistive layer 270 is formed on adhesion layer 265.Chemically resistive layer 270 is not attacked by agent 235. One or moreof layers 250, 255, 260 and 265 are optional, but layer 270 must bepresent. In the example that agent 235 is corrosive, chemicallyresistive layer 270 comprises Au or Pt or alloys of Au or alloys of Pt.In one example, adhesion layer 265 comprises Ti or Ta. In one example,water barrier layer 260 comprises Si₃N₄. In one example, stressabsorption layer 260 comprises HSQ, MSQ, polyphenylene oligomer,SiO_(x)(CH₃)_(y) or SiC_(x)O_(y)H_(y) or SiOCH, SiCOH), or porous SiCOH.In one example, buffer layer 250 comprises TEOS oxide.

FIG. 10 is a cross-section illustrating the action of anti-reverseengineering structures according to embodiments of the present inventionwhen the semiconductor chip is delayered from the top surface down. InFIG. 10 semiconductor chip 100 will be used as an example. Upondelayering from the top of semiconductor chip 100 to dielectric layer160, cap 225 (see FIG. 1) will be removed and agent 235 will be releasedinto regions 275. In regions 275, one or more of dielectric layers 110,120, 130, 140, 150 and 160 are not chemically inert to chemical agent235 (or are not chemically inert to chemical agent generated by thereaction of chemical agent 235 and air, oxygen or water) will be damagedor destroyed, one or more of wires 115, 125, 135, 145, 155 and 165 arenot chemically inert to chemical agent 235 (or are not chemically inertto chemical agent generated by the reaction of chemical agent 235 andair, oxygen or water) will be damaged or destroyed, or both one or moreof dielectric layers 110, 120, 130, 140, 150 and 160 and one or more ofwires 115, 125, 135, 145, 155 and 165 will be damaged or destroyedrending reverse engineering difficult or impossible.

FIG. 11 is a cross-section illustrating the action of anti-reverseengineering structures according to embodiments of the present inventionwhen the semiconductor chip is cross-sectioned. In FIG. 11 semiconductorchip 100 will be used as an example. Upon cross-sectioning from the sideof semiconductor chip 100 liner 220 will be breached and agent 235 willbe released into regions 280. In regions 280, one or more of dielectriclayers 110, 120, 130, 140, 150, 160 and 170 are not chemically inert tochemical agent 235 (or are not chemically inert to chemical agentgenerated by the reaction of chemical agent 235 and air, oxygen orwater) and will be damaged or destroyed, one or more of wires 115, 125,135, 145, 155, 165 and 175 will be damaged or destroyed, or both one ormore of dielectric layers 110, 120, 130, 140, 150, 160 and 170 and oneor more of wires 115, 125, 135, 145, 155, 165 and 175 are not chemicallyinert to chemical agent 235 (or are not chemically inert to chemicalagent generated by the reaction of chemical agent 235 and air, oxygen orwater) will be damaged or destroyed rending reverse engineeringdifficult or impossible.

Thus the embodiments of the present invention provide structures thatcomprise sealed trenches containing agents that will damage or destroyone or more of the materials in the various layers of the semiconductorchip (and/or the semiconductor substrate) by chemical attack physical orthermally stress when the seal around the trench is broken, therebypreventing reverse engineering or making reverse engineering verydifficult.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. An anti-reverse engineering semiconductor method,comprising: providing a semiconductor substrate; forming a stack ofwiring levels from a first wiring level to an intermediate wiring level,said first wiring level closest to said semiconductor substrate and saidintermediate wiring level furthest from said semiconductor substrate;forming active devices contained in said semiconductor substrate andsaid first wiring level, each wiring level of said stack of wiringlevels comprising a dielectric layer containing electrically conductivewire; forming one or more trenches extending from said intermediatewiring level, through said first wiring level into said semiconductorsubstrate; forming a liner on sidewalls and a bottom of said one or moretrenche such that said one or more trenches comprises an open space; and(i) filling said open space of said one or more trenches with a firstchemical agent that can cause damage to or destroy portions of at leastone wiring level of said stack of wiring levels or (ii) filling a firstgroup of said one or more trenches with a second chemical agent and afilling a second group of said one more trenches a third chemical agent,a mixture of said second chemical agent and said third chemical agentgenerating a fourth chemical agent that can cause damage to or destroysportions of at least one wiring level of said stack of wiring levels;forming a cap sealing a top of said open space of said one or moretrenches, wherein each of said liner and said cap is configured to bedamaged during a reverse engineering process such that said one or moretrenches is exposed to said at least one wiring level of said stack ofwiring levels; and forming one or more additional wiring levels on topof said intermediate wiring level.
 2. The method of claim 1, whereinsaid one or more trenches do not extend completely through saidsemiconductor substrate.
 3. The method of claim 1, wherein each of saidone or more trenches is filled with said first chemical agent and saidfirst chemical agent can attack wires, dielectric layers or both wiresand dielectric materials of said at least one wiring level of said stackof wiring levels.
 4. The method of claim 1, wherein said chemical agentgenerates a fifth chemical agent that can chemically attack wires,dielectric layers or both wires and dielectric materials of said atleast one wiring level of said stack of wiring levels when said chemicalagent is exposed to air, oxygen or water.
 5. The method of claim 1,wherein said mixture of second chemical agent and said third chemicalagent generates heat or wherein said mixture of second chemical agentand said third chemical agent generates a material that expands involume.